The mother died of natural causes, sickness.. You should probably go back 20-30 chapters of so to refresh your memory. Kamui never killed his mother.... Do you really think Kagura would be so desperate to save her brother if he actually her??
Oh, no sorry, what I was thinking about was actually from way way before this arc, I think, but I'm not really sure, that it was during the housen/ yoshiwara story that kagura said that kamui killed their mother. And if that were the case this whole thing now wouldn't make sense so that's why I think I'm remembering something wrong...
That never happen kagura said somewhere in the beginning that she was left all alone with her mother until she die of sickness and she buried her all by her self and since she got hungry she came to earth. It was never said that kamui killed her it was said that he try to killed his father but never her.
I guess I'm remembering this wrong but I always thought that it was said that kamui killed the mother ┑(￣Д ￣)┍